Token Ring local area network systems are well known in the prior art. A prior art Token Ring network typically includes a number of data stations or terminals connected sequentially together in a ring configuration via transmission cables. A coupling unit is typically provided in a Token Ring network for serially connecting a station into the network. When a new station needs to be connected into the ring network, the new station, when connected to its corresponding coupling unit, generates a DO configuration signal to the coupling unit. The coupling unit then uses the DC signal to effect a switching action to break the ring connection and serially connect the new station into the network at the breaking point. Cessation of the DC configuration signal typically causes a switching action in the coupling unit that bypasses the station and causes the station to be put in a looped (i.e., wrapped) state. This loop may be used by the station for the off-line self-testing functions.
The DC configuration signal does not affect the data transmission within the network because it is in DC form. Therefore, the DC configuration signal is transparent to the data transmission of the network. The DC configuration signal is also referred to as a phantom signal.
The phantom signal is typically generated by a phantom driver in the station. The phantom signal is applied to a phantom load which transfers the signal to the coupling unit. The phantom driver, however, needs to satisfy a number of requirements in order (1) to guarantee ring insertion of the station under non-fault conditions and (2) to detect and notify fault conditions. These requirements typically require the phantom driver to output a voltage not less than 4.1 volts for a DC current of zero to 1.0 milliampere. The phantom driver is also required to output a voltage not less than 3.9 volts for a DC current of 1.0 milliampere to 2.0 milliamperes. The phantom driver is also required to supply as a current source a DC current of no more than 20 milliamperes when the output of the phantom driver is short circuited. A phantom load having an impedance between 2.9K.OMEGA. ("kilo-ohms") and 5.5K.OMEGA. shall be detected as a non-fault condition. A phantom load having an impedance equal to or less than 0.1K.OMEGA. shall be detected as a short-circuit fault condition. A phantom load having an impedance equal to or greater than 9.9K.OMEGA. shall be detected as an open-circuit fault condition. A phantom load having an impedance between 0.1K.OMEGA. and 2.9K.OMEGA. or 5.5K.OMEGA. and 9.9K.OMEGA. shall be detected either as a fault condition or non-fault condition, depending on the implementation of the phantom driver.
An analysis of the above mentioned requirements indicates that the basic implementation of the phantom driver requires a resistor (i.e., resistor 21) connected to a voltage supply V.sub.CC, as illustrated in FIG. 1. As shown in FIG. 1, the output of phantom driver 20 is then connected to a phantom load 10 and a decoupling capacitor 11. Decoupling capacitor 11 is typically required by the Token Ring standard to be connected to the output of the phantom driver and ground. The resistance value of resistor 21 is required to be within the range of 200 .OMEGA. to 300 .OMEGA..
One implementation of resistor 21 is through n-well diffusion that diffuses the resistor into the well of a substrate. An n-well diffusion resistor typically has a resistance accuracy of approximately up to .+-.20%. Resistors fabricated by other implementations, such as polysilicon deposition, tend to have less resistance accuracy than that of the diffusion resistor.
Disadvantages are, however, associated with such phantom driver circuit. One disadvantage is that the resistance of resistor 21 is required to be relatively accurately set in order to guarantee adequate margin for high yield to the phantom load specifications. The lower the accuracy of the resistor, the lower the design margin for the phantom load. The resistance accuracy required for resistor 21 is typically better than.+-.15%. This indicates that resistor 21 of phantom driver 20 typically cannot be fabricated by the n-well diffusion or other known resistor fabrication processes that typically have less accuracy than the diffusion resistor.